In the relay optics of the space laser communication terminal's Acquisition, Pointing, and Tracking (APT) system, the Fiber Optic Nutator (FON), based on a Piezoelectric Ceramic Tube (PCT), is capable of actively achieving signal light reception and coupling through the implementation of energy feedback compensation algorithms with a lightweight design approach. Throughout the fiber nutation process, the deflection amplitude of the receiving fiber's end face significantly impacts the fiber coupling efficiency of the fiber optic nutator. To quantify this influence, the curve depicting the effect of the relative aperture (D/f) of the relay optics focusing lens on fiber coupling efficiency is initially computed. Notably, when D/f=0.213, the fiber coupling efficiency attains its theoretical maximum of 0.813. Subsequently, the composite motion of the fiber end face in three-dimensional space is deconstructed into radial and axial translations, along with rotations based on the axial direction. Through meticulous simulation calculations, it is ascertained that the fiber coupling efficiency decreases by more than 5% when the radial displacement r of the fiber end face exceeds 3.65μm, or when the axial displacement d surpasses 0.25mm, or when the angular deviation θ exceeds 0.08°. These findings offer quantifiable criteria for the dimensional selection of the PCT under varied application conditions, providing constructive guidance for determining core structural design parameters of the fiber optic nutator. © COPYRIGHT SPIE.